We are really looking for points that are inside of a circle of center (x,y) and radius d.
The square that encloses circle is a square of center (x,y) and sides 2d. Any point out of this square does not need to be checked, it's out. So, a point a (xa, ya) is out if abs(xa - x) > d or abs (ya -yb) > d.
Same for the square that is enclosed by that circle is a square of center (x,y) and diagonals 2d. Any point out of this square does not need to be checked, it's in. So, a point a (xa, ya) is in if abs(xa - x) < (d * 1.412) or abs(ya -yb) < (d * 1.412).
Those two easy rules combined reduce a lot the number of points to be checked. If we sort the point by their x, filter the possible points, sort by their y, we come to the ones we really need to calculate the full distance.